The present invention relates generally to a nacelle for protecting a reusable rocket engine from the effects of its exhaust plume and, more particularly, to a nacelle which includes a rigid shroud enclosing and rotating with the rocket engine when the engine is rotated to vector its thrust.
The exhaust plume emananting from the nozzle of a reusable liquid propellant rocket engine creates a severe thermal and acoustic environment adjacent to the engine. More particularly, the temperature adjacent the nozzle""s exhaust orifice may exceed two thousand degrees Fahrenheit (2,000xc2x0 F.). At high altitudes, the exhaust plume expands laterally beyond the circumference of the nozzle""s exhaust orifice. Thus, especially at high altitudes, the lateral sides of the engine, if left unprotected, would be exposed to an extremely high heat transfer rate due to convection and radiation from the expanded exhaust plume. Many of the essential components of a liquid propellant rocket engine, in particular, the lines communicating the liquid propellant, hydraulic fluid, and electrical current, could not withstand such a high heat transfer rate.
The exhaust gases expand downstream of the choke plane of the nozzle. A lattice of standing shock waves is created in the exhaust plume when those gases expand and accelerate to reach supersonic velocity. High amplitude acoustic waves are generated by the shock waves in the plume. When the flight vehicle is in subsonic flight, these acoustic waves travel upstream from the plume. If the rocket engine was left unprotected, these waves would impinge on the engine""s sides. Due to the proximity of the rocket engine to the exhaust plume, the strength of these acoustic waves would be only minimally diminished upon impingement. Repeated exposure to such high intensity acoustic waves would cause fatigue in the operating parts and structure of the rocket engine, and deleteriously affect its reliability and structural integrity.
Furthermore, in a vertical launch the exhaust plume causes debris on the ground to be blown upwards. Absent a protective barrier, this debris could impinge on the sides of the rocket engine. This problem would also be present should a liquid propellant rocket engine be used on a flight vehicle designed to land vertically, that is, with the thrust vector oriented perpendicularly to the ground, as opposed to a conventional horizontal landing.
Designers of reusable rocket engines have used rigid covers to insulate the engines from the heat and acoustic waves generated by the exhaust plume, as well as to protect them from impinging debris blown upwards from the ground during launch. Since modern rocket engines rotate about at least one axis in order to vector thrust, the cover must allow for such rotation. In addition, since the rocket engine is reusable, the cover must provide for easy access to facilitate inspection, maintenance and repair of the engine.
As illustrated in FIG. 1, one approach has been to enclose rocket engine 11 with rigid cover 13, and to attach cover 13 to flight vehicle 15. In particular, rocket engine 11 includes gimbal 17, propellant lines 19 communicating with powerhead 21, exhaust nozzle 22, and nozzle throat 23. Nozzle 22 includes insulation 25 to protect its exterior sides from the acoustic waves and heat that will emanate from an expanded exhaust plume.
Gimbal 17 is attached to flight vehicle 15 and allows rocket engine 11 to rotate relative to flight vehicle 15 about pitch and yaw axes. Pitch actuator 27 is connected to engine 11 and rotates it about the pitch axis to vector its thrust. A second actuator for controlling yaw rotation is not shown. Powerhead 21 contains electrical, hydraulic, and liquid propellant lines.
Cover 13 is fixedly attached to flight vehicle 15 and is comprised of shroud 29 and eyeball shield 31. Eyeball shield 31 includes annular opening 33, which circumscribes nozzle throat 23. Flexible annular seal 35 provides an airtight interface between shroud 29 and eyeball shield 31. Eyeball shield 31 rotates with rocket engine 11 as the engine is rotated about gimbal 17 by pitch actuator 27 and the yaw actuator. Eyeball shield 31 thus rotates relative to flight vehicle 15 and shroud 29, along with rocket engine 11.
A second approach is shown in FIG. 2. Rocket engine 37 includes gimbal 39, propellant lines 41 communicating with powerhead 43, exhaust nozzle 45, and nozzle exhaust orifice 47. Gimbal 39 is attached to flight vehicle 49 about pitch and yaw axes. Actuator 51 is connected to rocket engine 37 and rotates it about the pitch axis to vector its thrust. A second actuator for controlling rotation about the yaw axis is not shown.
Cover 53 is rigidly attached to flight vehicle 49 and is comprised of shroud 55 and eyeball shield 57. Eyeball shield 57 includes annular opening 59. In this case however, opening 59 circumscribes exhaust orifice 47 rather than the nozzle throat. Annular seal 61 provides an airtight interface between shroud 55 and eyeball shield 57. Eyeball shield 57 thus rotates relative to vehicle 49 and shroud 55, along with rocket engine 37.
Annular seals 35 and 61 are comprised of a complex spring mechanism which presses a flexible material against the opposing surfaces of the shroud and the sliding eyeball shield. Due to their mechanical complexity and the effect of harsh operating conditions, the annular seals used in the engine covers of the prior art are expensive, unreliable, and require continual inspection, adjustment, and maintenance.
Furthermore, inspecting and performing repairs or routine maintenance on the rocket engine requires removing and reinstalling the annular seal of the prior art because the eyeball shield remains attached to the nozzle when the shroud is removed. Referring to rocket engine 37 in FIG. 2, eyeball shield 57 remains attached to exhaust orifice 47 when shroud 55 is removed from flight vehicle 49. Removing shroud 55 pursuant to performing routine maintenance on engine 37 thus entails detaching shroud 55 from flight vehicle 49 and removing seal 61. Reinstallation of shroud 55 requires reinstallation of seal 61, which is a tedious, laborious and time consuming task.
Based on the foregoing, it can be appreciated that there presently exists a need in the art for a nacelle for a reusable rocket engine which overcomes the above-described disadvantages and shortcomings of the prior art. The present invention fulfills this need in the art.
Briefly, the present invention encompasses a nacelle for enclosing a reusable liquid propellant rocket engine. The nacelle is comprised of a rigid shroud and a support truss. The shroud encloses the truss and the rocket engine. The shroud is comprised of three sections, including a top section and two side sections. The top section has openings to permit the communication of lines for liquid propellant, electrical current, and hydraulic fluid between the flight vehicle and the engine. The two side sections are connected to each other by longitudinal field joints, and to the top section by a circumferential field joint.
The rocket engine is attached to the flight vehicle by a gimbal allowing the engine to rotate relative to the flight vehicle about pitch and yaw axes. A pair of actuators is located in the flight vehicle. One of the actuators controls the rotation of the engine about the pitch axis, while the other controls the rotation about the yaw axis. Each actuator is connected to the shroud at a hard point where the shroud is attached to the truss.
The shroud is also fastened to the rocket engine around the nozzle""s exhaust orifice, and around an attachment cone located adjacent to the gimbal. The foregoing attachment configuration transmits almost all of the forces applied by the actuators through the truss to the engine. The shroud transmits very little of the actuator forces. Since the shroud is attached to the rocket engine, the shroud rotates with the engine.
The nacelle of the present invention insulates the propellant, hydraulic and electrical lines of an enclosed rocket engine from the severe thermal and acoustic environment occasioned by the exhaust plume, as well as protects them from ground debris blown upwards during launch and, possibly, landing. It achieves the foregoing without using the complex spring-loaded seals of the prior art. The nacelle is thus able to realize improved reliability, savings in cost, and reduced maintenance over the engine covers of the prior art. Furthermore, removal and installation of the nacelle takes substantially less time and labor compared to the prior art covers. This significantly facilitates inspecting, maintaining and repairing the reusable rocket engine.